1. Technical Field
Exemplary aspects of the present disclosure generally relate to an image forming apparatus, such as a copier, a facsimile machine, and a printer.
2. Description of the Related Art
There has been known a multicolor image forming apparatus in which color images of four different colors, i.e., cyan, magenta, yellow, and black are superimposed one atop the other to form a multicolor image. A tandem-type image forming apparatus is a mainstream multicolor image forming apparatus in recent years. In general, the tandem-type image forming apparatus includes four drum-shaped image bearers, one for each of the colors cyan, magenta, yellow, and black to form toner images. The image bearers are arranged in tandem along an image formation path.
In the image forming apparatus of this kind, an intermediate transfer belt is disposed contacting the image bearers from which the toner images are transferred onto the intermediate transfer belt one atop the other to form a composite toner image in a process known as primary transfer. The composite toner image on the intermediate transfer belt is transferred onto a transfer sheet or a recording medium supplied from a paper feed unit at a contact portion at which the transfer sheet contacts the intermediate transfer belt in a process known as secondary transfer.
Primary transfer devices are disposed inside the looped intermediate transfer belt opposite the respective image bearers via the intermediate transfer belt at the primary transfer portion. As the primary transfer devices, rollers (hereinafter referred to as primary transfer rollers) are commonly employed. A voltage is supplied to the primary transfer rollers, thereby transferring the toner image onto the intermediate transfer belt.
The intermediate transfer belt is looped around a pulley (i.e., a secondary-transfer opposed roller), and a secondary transfer device is disposed outside the loop formed by the intermediate transfer belt, opposite to the secondary-transfer opposed roller at a secondary transfer portion. As the secondary transfer device, a roller (hereinafter referred to as a secondary transfer roller) is commonly employed. An electric field is generated between the secondary transfer roller and the secondary-transfer opposed roller, thereby transferring the toner image from the intermediate transfer belt onto a recording medium.
In the image forming apparatus of this kind, it is important to move the surfaces of the image bearers and the intermediate transfer belt at a constant speed. Fluctuations in the surface moving speed of the image bearer causes stretching and shrinkage of an image. Even a slight fluctuation may cause irregular image density. Furthermore, even when the surface moving speed of the image bearer is constant, fluctuations in the traveling speed of the intermediate transfer belt cause a difference between the moving speed of the image bearer and the intermediate transfer belt, causing also stretching and shrinkage of an image.
An example of a cause of fluctuations in the traveling speed of the intermediate transfer belt includes the use of a thick sheet as a recording medium. When a relatively thick sheet enters a secondary transfer nip at which the secondary transfer roller and the secondary-transfer opposed roller meet and press against each other at the secondary transfer portion, load on the secondary-transfer opposed roller changes in order to introduce the leading edge of the thick sheet into the secondary transfer nip, which causes an instantaneous change in the speed. This causes fluctuations in the traveling speed of the intermediate transfer belt, which results in irregular image density of the image on the intermediate transfer belt during primary transfer.